1. Field of the Invention
This invention generally relates to the measurement of power consumption and more specifically to non-intrusive and self-powered measurement of the power factor of a circuit.
2. Prior Art
In a typical electricity distribution system, power is provided through a main circuit breaker and a device for measurement of the power consumption of the entire electrical network connected thereto. However, typically, the main power line is then connected to a plurality of circuit breakers, each feeding a smaller section of the electrical network with its specific power requirements. The circuit breaker is adjusted to the amount of maximum current that may be used by this electrical sub-network. In industrial and commercial applications, hundreds of such circuit breakers may be installed, each controlling a section of the electrical network. Even in smaller locations, such as a house, it is not unusual to find tens of circuit breakers controlling various electrical sub-networks.
Non-intrusive measurement of current through a power line conductor has well known principles. A current transformer (CT) of sorts is created that comprises the primary winding as the power line conductor and the secondary providing an output current inversely proportionate to the number of windings. Typically such systems are used for measuring currents in very high voltage or current environments, for example, as shown in Gunn et al. in U.S. Pat. No. 7,557,563. These types of apertures are useful for main power supplies. Using such devices, or power meters for that matter, is deficient for the purposes of measuring relatively low currents in an environment of a plurality of circuit breakers. Providing wireless telemetry on a singular basis, such as suggested by Gunn et al., and other prior art solutions, suffers from deficiencies when operating in a noisy environment.
The power factor (cosφ) of an alternate current (AC) is defined as the ratio between the real power and the apparent power of the circuit, where the apparent power is the average voltage multiplied by the average current consumed by the load, and the active power is the power which is actually consumed by the load, taking into consideration the loss of reactive energy oscillating between the network and the device. The power factor is therefore a number between 0 and 1, having a value of 1 for a purely resistive load that has a zero phase shift of the current with respect of the voltage. However typical loads are not purely resistive, as the current is distorted and contains higher frequency harmonics, as well as a phase shift relative to the pure voltage signal. As a result the power factor is lower than 1. Commercially available metering devices are often bound to standards and customer requirements enforcing measurement of the power factor in order to calculate the actual power consumed.
Measuring the power factor of a circuit requires simultaneous measurement of both voltage and current. Existing sub-metering solutions, that are systems designed to meter electricity at several branches of the electric grid inside a building, typically comprise of a meter mounted inside or next to an electric circuit breaker closet, also referred to at times as a panel-board. The meter is hardwired to the voltage line in order to measure the voltage, and is also hardwired to one or more current transformers (‘CT’) for the purpose of measuring the current in one or more locations in the panel board. The major drawback of this approach is observed when the number of circuits to be measured, i.e., the sub-metering granularity, increases, as more CT's must be wired inside the panel-board. Wiring becomes intensive and the cost and invasiveness of the installation increases. As sub-meters need to be hardwired both to the voltage and CT's at least one meter is needed in each panel. As sub-meters are typically limited by the number of CT's they can handle, it may be necessary to have more than a single meter in each panel board.
There is a need in the art that is now developing, resulting from the move toward energy conservation to enable measurement and analysis of the power factor of a circuit grid on a finer granularity. It would be advantageous if a solution may be provided for installation of such a solution in a panel board for existing circuit breakers. It would be further advantageous to enable a fine granularity of measurement of the current without causing congestion in the panel board. It would be therefore beneficial to overcome the limitations of the prior art by resolving these deficiencies.